Abstract/Summary

Long term ringing of chicks and retrapping of adult Shags at a colony in south-east Scotland have provided information on survival to breeding age for 25 cohorts. The mean survival for 18 cohorts at risk of being recaptured for 10 years was 0.307 +/- 0.2. Generalized linear modelling demonstrated that survival varied according to year, fledging date and brood size. The effect of each variable was apparently independent of those of the other two. Young hatched early in the season survived better than those hatched later which, when linked to the seasonal decline in breeding success, results in early laying individuals contributing 10 times as many recruits to subsequent generations as do birds laying two months later. Chicks in broods of three had a higher estimated survival than those in broods of two and both appeared to survive very much better than those in broods of one. Differences in the survival of cohorts could not be explained by annual differences in the timing of breeding or mean brood size. Rather, differential survival of cohorts appeared to be due to the timing of mortality during the first winter.